A resin material is widely utilized for producing a variety of molded products such as interior and exterior members of automobiles, exterior members of household electric goods such as refrigerators and washing machines, trays, shelves, and wrapping sheets. It is generally known that such physical properties of articles of resin material as rigidity and impact strength are improved by incorporating such additives as inorganic fillers, lubricants and elastomers into resin material. Examples of the inorganic fillers include fibrous inorganic fillers such as fibrous basic magnesium sulfate and non-fibrous inorganic fillers such as talc.
Generally, a resin material containing an inorganic filler is produced by preparing in advance a master batch (resin composition) containing the inorganic filler in a high concentration and then melt-kneading the master batch with an additional resin material, so as to obtain the desired resin composition (target product).
JP 2006-83369 A describes a process for producing a polypropylene resin composition containing a fibrous inorganic filler, a non-fibrous inorganic filler and elastomer, which comprises the first step for preparing a resin composition (MB) by melt-kneading a first propylene polymer (A-I) and a fibrous inorganic filler (B) in a weight ratio of 3/7 to 7/3 (A-I/B) and the second step for preparing a resin composition by melt-kneading the resin composition (MB) prepared in the first step, after addition of a second propylene polymer (A-II), a non-fibrous inorganic filler (C) and an olefinic elastomer and/or a vinyl aromatic compound-containing elastomer (D), to obtain a desired resin composition. This publication describes a fibrous magnesium oxysulfate (fibrous basic magnesium sulfate) as an example of the fibrous inorganic filler. The publication also describes a single screw extruder, a double screw extruder, a Banbury mixer, and a heated roller, as examples of the extruders for kneading the propylene polymer (A-1) and fibrous inorganic filler (B).
Such extruders as those described in the above-mentioned publication are generally equipped with a hopper for the supply of the materials (starting materials) for mixing. Therefore, a melt-kneaded product (resin composition) comprising propylene polymer and fibrous basic magnesium sulfate is generally produced by the steps of supplying a mixture which is obtained by uniformly mixing polyolefin particles and fibrous basic magnesium sulfate aggregated particles in advance into a supply port of the hopper of the melt extruder, dropping the mixture within the hopper so as to deposit the mixture in the bottom thereof, and finally incorporating the mixture into the melt extruder so as to melt-extruding the mixture.
A various kinds of polyolefin particles as well as fibrous basic magnesium sulfate aggregated particles having different mean particle sizes are now produced. In addition, the fibrous basic magnesium sulfate aggregated particles having different apparent density are now produced by adjusting the aggregation conditions. The adjustment of the aggregation conditions can be done by varying the granulating conditions and the drying conditions.